Molecular hydrogen ina-Si: H

Recently Conradi and Norberg have proposed that a small density of molecular hydrogen in $a$-Si: H films provides the relaxation mechanism which is responsible for a minimum in the proton spin-lattice relaxation time ${T}_{1}$ at about 30 K. Although we are unable to observe an NMR line attributable to the ${\mathrm{H}}_{2}$, we are able to observe the conversion of the ${\mathrm{H}}_{2}$ molecules from the ortho state to the para state at 4.2 K. The process is bimolecular with a rate constant of 0.010 ${\mathrm{h}}^{\ensuremath{-}1}$. The existence of a large number of sites able to trap such a small molecule may provide an important insight into the defect structure of these films.